It is generally believed that the stimulus for smooth pursuit eye movements is motion of a target relative to the retina. The proposed research will show that not only target velocity but also target position (offset from the fovea) can elicit pursuit movements, and that position the more important stimulus. In accord with others, we will show that once a pursuit movement begins, the eye movement itself contributes to the perceived motion of the target. Further, we will show that for a constant retinal target motion, an increase in the perceived target motion produces and increased stimulus to the pursuit system, resulting in an increased response. The experiments form a basis for the hypothesis that a loop is formed by eye movement generating increased perception of target motion, in turn generating further eye movement, and that this positive feedback system keeps the eye on a moving target. The above experiments will be performed in "open-loop" conditions, i.e. with the stimulus stabilized on the retina so that the retinal stimulus is independent of the eye movements. To study responses to perceived motion, we will create or modify apparent target motion by variations of the Duncker illusion. (In this illusion, motion of a large frame causes the target to appear to move in the opposite direction.) To study responses to target position, the target will be stepped to various extrafoveal positions. We will develop and apply a technique for studying responses of the pursuit system in normal ("closed-loop") conditions, but without the usual interference from saccadic eye movements when the target moves a substantial distance off of the fovea. This technique depends on detection of saccades and stabilization of the stimulus against them. We will perform experiments to assess the significance of the predictability of target motion to the pursuit system: by studying ballistic attempts to acquire fixation of targets seen for a variable period, we will develop a quantitative measure of the time course of the pursuit system developing a prediction of future target behavior.